OBJECTIVE: The long-term objective of this project is the construction of an analog tissue densitometer suitable for incorporation into routine radiotherapy-planning procedures. The principles behind the proposed system have already been verified by a pilot project in which performance close to that of the EMI scanner was achieved. APPROACH: (1) The system will provide transaxial tomograms that will permi accurate localization of a lesion; (2) it will provide numerical data input to a conventional therapy-planning computer; and, (3) it will provide an analog feedback signal to dynamically control the energy or beam current of a therapy machine. This latter application is practical because the low cost of the data-processing subsystem permits it to be dedicated to a particular therapy machine. Other advantages of the proposed system include simplicity, good resolution and speed of both data aquisition and data processing. There is also the possibility of mounting the detector assembly on the treatment accelerator gantry and obtaining images with either the treatment beam itself or an auxiliary X-ray tube mounted on the gantry.